FT-817 Zero Beat, and
switching
between CW
and SSB modes

This information
is adapted from a posting titled "Re: Zero Beat confusion" which I
emailed to the FT-817
group
on Saturday 09 October 2010 .The original question
asked about the best way to "zero beat" another station when operating
CW with the FT-817. (This can be achieved by pressing and holding
the HOME button on the front panel of the FT-817 - this causes the
sidetone to be heard in the speaker (or headphones) at the same time as
the received signal - while adjusting the tuning knob until the
sidetone has the same note as the received CW signal. When doing
this, listen for
the beat note between the sidetone and the received signal - this will
reduce in frequency as the tuned frequency approaches the frequency of
the received CW signal).

Subsequent discussion extended the original question to related issues;
such as the actual transmit frequency when operating in CW or CW-R
mode; the difference between these two modes; and why does the
displayed frequency change when switching between CW, CWR, USB and LSB
modes. My posting attempted to provide some answers to these
latter questions.

To illustrate
what the FT-817 does when switching
between CW, CWR, USB and LSB modes, I
used the FT-817 to listen to the
VK2WI CW practice
beacon transmitted from Dural (near Sydney, NSW) on 3699 kHz.
My
FT-817 is configured for a 800 Hz CW
pitch, and is not fitted with a narrow filter for CW reception.
For this experiment, the clarifier and IF shift were off.

(Note that you
can conduct the same experiment with your own FT-817 even if you're not
within hearing distance of the VK2WI CW practice beacon. All you
need is a medium to strong CW signal which is transmitting continuously.)

CW Mode

Firstly, I
selected CW mode, and tuned the FT-817 to 3699 kHz. The front panel
display indicates this as "3.699.00". Pressing and
holding the HOME button confirmed that the received signal had the same
pitch as the sidetone, therefore the displayed frequency was that of
the received CW signal. The diagram below illustrates the radio
frequency spectrum (as "seen" at the RF front end of the FT-817), and a
representation of the FT-817 front panel display.

The "Radio
Frequency Spectrum" section of the diagram above shows the passband of
the IF filter, but referenced to the frequencies "seen" at the receiver
RF front end. The passband is shown as being 3 kHz wide, which is
an approximation of the published IF filter bandwidth. (IF filter
bandwidths are often specified at the -6dB and -60dB points of the
filter response; however, when used in practice, the filter passband
sounds as if it is wider than the -6dB bandwidth but narrower than the
-60dB bandwidth - this is because the human ear can hear signals below
the -6dB level.) The CW signal from VK2WI beacon is shown as
falling within this passband, and this is suggested by the "S+" and
appropriate signal strength bars shown on the FT-817 display. The
FT-817 internally tunes the Beat Frequency Oscillator (BFO) so that it
is below the received signal frequency by the same amount as the CW
pitch, ie 800 Hz. This is achieved by tuning the FT-817 so that
the display shows the transmitted frequency of the desired station - in
this case, 3699 kHz for VK2WI's CW beacon signal. If I was to
transmit with the FT-817 in this configuration (which is not
recommended, as my transmission would interfere with the beacon
transmission), then the CW transmission would occur on 3699 kHz.

CWR Mode

Next, I switched
to CWR mode. The FT-817 displayed "3.699.00", and the
received signal had the same pitch. The diagram below illustrates
what happened.

This diagram is
very similar to that shown for the CW mode. The CW signal from
VK2WI beacon
is shown as falling within the filter passband, and this is indicated
by the
"S+" and appropriate signal strength bars shown on the FT-817
display. However, this time
the BFO is tuned so that it is
above the received signal frequency by the same amount as the CW pitch,
ie 800 Hz. Note that no retuning of the FT-817 was
required.
Again; if
I
was
to
transmit
with
the
radio
in
this configuration (which is not
recommended for the reason given above), then the CW transmission would
occur on 3699 kHz.

USB Mode

Without touching
the tuning knob, I switched to USB
mode. The FT-817 now displayed "3.698.20", and the
received signalpitch had not
changed. The difference between the frequency displayed
for CWand CWR, and the
frequency displayed for USB, is 800 Hz - the sidetone
pitch for my
FT-817.The
diagram
below
illustrates
what
happened.

The
diagram above is similar to that shown for the CW and CWR modes. The CW signal from
VK2WI beacon
is shown falling within the filter passband, and this is indicated by
the
"S+" and appropriate signal strength bars shown on the FT-817
display. However, this time
the BFO is tuned so that it is
below the received signal frequency by the same amount as the CW pitch,
ie 800 Hz. The BFO being tuned to a frequency just below the
passband is expected for USB mode - it is equivalent to the suppressed
carrier associated with what amateur radio operators called
"SSB". (The mode is more correctly known as "Single Sideband -
Suppressed Carrier", or SSB-SC.) If I was to transmit
with the radio in this configuration (which is not
recommended for the reason given above), then my SSB signal would fall
within the band between 3698.2 kHz and 3701.2 kHz, and interfere with
the VK2WI CW beacon signal. Also, the upper 1.2 kHz of my USB
signal would fall outside the 80m band allocated for use by amateur
radio operators in Australia, so it would not be a legal transmission!

LSB Mode

Then, I switched
to LSB mode. The '817 displayed "3.698.20", and the VK2WI
signal could not be heard. This is because the signal is at 3.699.00,
and the FT-817 is receiving signals falling in the 3 kHz wide passband
between
3.698.20 and 3.695.20. The diagram below illustrates what
happened.

The
diagram is similar to that shown above for the CW, CWR and USB
modes. However, the CW signal from
VK2WI beacon
is shown as falling outside the filter passband, and this is indicated
by the
"S1" and appropriate signal strength bar shown on the FT-817
display. (S1 was the level of some in-band noise and other weak
signals at the time.) The BFO is tuned for
receiving LSB signals in the band between 3698.2 kHz and 3695.2 kHz. The BFO being
tuned to a frequency just above the passband is expected for LSB mode -
it is
equivalent to the suppressed carrier associated with LSB. If I was to transmit
with the radio in this configuration, then my LSB signal would not
interfere with the VK2WI CW beacon at all.

Retuning the
FT-817 (still in LSB
mode) to 3699.8 kHz brought the
VK2WICW
signal back to zero beat, ie 800 Hz pitch, as shown in the diagram
below.

This
diagram is similar to that shown above for LSB. In this case, the CW signal from
VK2WI beacon
is shown falling within the filter passband, and this is indicated by
the
"S+" and appropriate signal strength bars shown on the FT-817
display. However, this time
the BFO is tuned so that it is
above the received signal frequency by the same amount as the CW pitch,
ie 800 Hz. Note that the '817 had to be retuned to a frequency
800 Hz above VK2WI CW beacon transmit frequency in order to obtain the
800 Hz pitch. If I was to transmit
with the radio in this configuration (which is not
recommended for the reason given above), then my LSB signal would fall
within the band between 3699.8 kHz and 3696.8 kHz - thus causing
interference to the beacon transmission.

Comparisons
between SSB (USB and LSB) and CW (CW and CWR) modes

Given the
discussion above regarding CW, CWR, USB and LSB modes; it is
interesting to compare the CW and USB modes, and the CWR and LSB
modes. It is also interesting to compare LSB and USB modes.

USB & CW
modes

The diagram
above shows the radio frequencies at the RF front end of the FT-817's
receiver, and the representation of the FT-817's front panel display,
for USB and CW modes. Note that the
IF filter passband, when referenced to the receiver RF front end,
covers the same frequency band regardless of the mode. Also note
that the BFO is at the same frequency, regardless of the mode. In
USB and CW modes, the FT-817 implements a USB receiver. This can
be verified by
selecting either mode and listening to a CW signal - increasing the
received frequency decreases the pitch of the
received CW signal; and conversely decreasing the received frequency
will increase the pitch of the received CW signal. That is
exactly what is expected of a USB receiver.

The
frequency displayed on the front panel depends on the mode. This
is because in CW mode, the displayed frequency is the frequency on
which the CW transmission will occur; whereas in USB mode, the
displayed frequency is the frequency of the suppressed carrier.
(The carrier is suppressed in the balanced modulator.)

LSB and CWR
modes

The
diagram
above shows the radio frequencies at the RF front end of the FT-817's
receiver, and the representation of the FT-817's front panel display,
for LSB and CWR modes. Note that the IF filter passband, when
referenced to
the receiver RF front end, covers the same frequency band regardless of
the mode. Also note that the BFO is at the same frequency,
regardless
of the mode. In LSB and CWR modes, the FT-817 implements a LSB
receiver. Again; this can be verified
by selecting either mode and listening to a CW signal - increasing the
receivedfrequency
increases
the
pitch
of
the
received
CW
signal;
and conversely decreasing the
received frequency decreases the pitch of the
received CW
signal. That
is
exactly
what
is
expected
of
a
LSB
receiver.

Again,
the
frequency displayed on the front panel depends on the mode. This
is
because in CW mode, the displayed frequency is the frequency on which
the CW transmission will occur; whereas in LSB mode, the displayed
frequency is the frequency of the suppressed carrier. (Per the
USB case, the carrier is
suppressed in the balanced modulator.)

USB and LSB
modes

The diagram
above shows the radio frequencies at the RF front end of the FT-817's
receiver, and the representation of the FT-817's front panel display,
for USB and LSB modes. Note that the IF filter passband, when
referenced to
the receiver RF front end, effectively moves from one side of the BFO
(or suppressed carrier frequency) to the other. For a symmetrical
filter passband, this can be thought of as the filter passband being
"mirrored" around the BFO frequency. Also note that the BFO is at
the same frequency, regardless
of the mode. In USB mode, the
receiver is listening to a 3 kHz segment of the band
extending from the suppressed carrier frequency up
to afrequency
3
kHz
greater
than
that
suppressed
carrier
frequency;
and when in LSB
mode,
the receiver
is
listening
to
an
equivalent
band
extending
from
the
suppressed carrier frequency
down to a frequency 3 kHz less than that suppressed carrier
frequency. This is well
illustrated when listening to a DSB signal - when the
suppressed carrier frequency in
either LSB or USB mode is correctly tuned to the
DSBsuppressed
carrier
frequency,
then
switching
between
LSB
and
USB
will
not cause any
difference
-
the
received
signal
is
still
intelligible
(ignoring
any effectsof QRM, etc, on one
sideband and not the other). In the diagram above, the indicated
signal strength in USB mode is higher due to the CW carrier and DSB
upper sideband being received, whereas in LSB mode only the DSB lower
sideband is received.Back to top

This page was created by Mike
Dower
VK2IG:
24
Dec 2010; last
updated: 26 Mar 2011. Material may be copied for personal or
non-profit use
only.